Shield assignment using preferential shields

ABSTRACT

A method for preferentially shielding a signal to increase implicit decoupling capacitance is provided. The signal is preferentially shielded by using a probability of the signal being at a specific value to assign a shield potential. Further, an integrated circuit that preferentially shields a signal to increase decoupling capacitance by using a probability of the signal being at a specific value to assign a shield potential is provided. Further, a computer system for preferentially shielding a signal to increase decoupling capacitance by using a probability of the signal being at a specific value to assign a shield potential is provided. Further, a computer readable medium having executable instructions for preferentially shielding a signal to increase implicit decoupling capacitance by using a probability of the signal being at a specific value to assign a shield potential is provided. Further, a method to increase system performance by increasing implicit decoupling capacitance is provided.

BACKGROUND OF INVENTION

[0001] A typical computer system has at least a microprocessor and memory. The microprocessor processes, i.e., executes, instructions to accomplish various tasks of the computer system. Such instructions, along with the data required by the microprocessor when executing these instructions, are stored in some form of memory. FIG. 1 shows a typical computer system having a microprocessor (10) and some form of memory (20). The microprocessor (10) has, among other components, a central processing unit (also known and referred to as “CPU” or “execution unit”) (12) and a memory controller (also known as “load/store unit”) (14). The CPU (12) is where the actual arithmetic and logical operations of the computer system take place. To facilitate the execution of operations by the CPU (12), the memory controller (14) provides the CPU (12) with necessary instructions and data from the memory (20). The memory controller (14) also stores information generated by the CPU (12) into the memory (20).

[0002] The operations that occur in a computer system, such as the logical operations in the CPU and the transfer of data between the CPU and memory, require power. If the components responsible for carrying out specific operations do not receive adequate power in a timely manner, computer system performance is susceptible to degradation. As an added challenge, power consumption of modern computers has increased as a consequence of increased operating frequencies. Thus, providing power to the components in a computer system in a sufficient and timely manner has become an issue of significant importance.

[0003] Often, power supplied to a computer system component varies, which, in turn, affects the integrity of the component's output. Typically, this power variation results from the distance between a power supply for the component and the component itself. This distance may lead to the component not receiving power (via current) at the exact time it is required. One approach used by designers to combat this performance-inhibiting behavior is introducing decoupling capacitance (also referred to as “decap”) to a particular circuit by positioning one or more decoupling capacitors close to the component. These decoupling capacitors store charge from the power supply and distribute the charge to the component when needed. For example, if power received by a component from a power supply attenuates, one or more decoupling capacitors will distribute charge to the component to ensure that the component is not affected by the power variation on the power supply. In essence, a decoupling capacitor acts as a local power supply for one or more specific components in a computer system.

[0004] Within a computer system component, such as a circuit, there are two types of decoupling capacitance: implicit and explicit. Explicit decoupling capacitance is provided to the circuit through the use of decoupling capacitors as discussed above. Implicit decoupling capacitance (also known in the art as “parasitic capacitance” or “inherent capacitance”) is capacitance that is inherent in a circuit. Implicit decoupling capacitance results from the electromagnetic effects between current-carrying wires. Further, implicit decoupling capacitance is a function of the distance between two such wires. Also, the ability to help supplement an attenuating voltage using explicit decoupling capacitors or implicit decoupling capacitance is a function of the potential applied to the decaps.

[0005]FIG. 2 shows the presence of explicit and implicit decoupling capacitance in a section of a typical computer system component (40). The component (40) has a power supply bus (44) and a ground bus (46) that provides power through a connection to a power supply (42). The power supply (42) may be a part of the component (40) or a separate element. Power from the power supply (42) is made available to multiple power supply lines (48) and (52) via connections to the power supply bus (44) and to multiple ground lines (50) and (54) via connections to the ground bus (46). Power from the power supply (42) is delivered to chip logic circuits (60) and (68) via the power supply lines (48) and (52), respectively, and ground lines (50) and (54), respectively. When there is power variation across the power supply (42), explicit decoupling capacitors (56), (57), (58), and (59) positioned in parallel with the power supply (42) provides charge, i.e., power, to the chip logic circuits (60) and (68).

[0006] Still referring to FIG. 2, the existence of implicit decoupling capacitances (64), (66), (72), and (74) is shown. A first occurrence of implicit decoupling capacitance (64) occurs between the power supply line (48) and a signal line (62) from the chip logic (60). A second occurrence of implicit decoupling capacitance (66) occurs between the signal line (62) and the ground line (50). The implicit decoupling capacitances (64) and (66) are dependent on the characteristics of the signal line (62), specifically, whether a signal on the signal line (62) is high or low. When the signal is low, the implicit decoupling capacitance provided to the chip logic (60) is equal to the implicit decoupling capacitance (64) between the power supply line (48) and the signal line (62). Alternatively, when the signal is high, the implicit decoupling capacitance provided to the chip logic (60) is equal to the implicit decoupling capacitance (66) between the signal line (62) and the ground line (50).

[0007] Still referring to FIG. 2, implicit decoupling capacitance is also present in a substantial number of additional circuits. For example, another first occurrence of implicit decoupling capacitance (72) occurs between the power supply line (52) and a signal line (70) from the chip logic (68). Another second occurrence of implicit decoupling capacitance (74) occurs between the signal line (70) and the ground line (54). The implicit decoupling capacitances (72) and (74) are dependent on the characteristics of the signal line (70), specifically, whether a signal on the signal line (70) is high or low. When the signal is low, the implicit decoupling capacitance provided to the chip logic (68) is equal to the implicit decoupling capacitance (72) between the power supply line (52) and the signal line (70). Alternatively, when the signal is high, the implicit decoupling capacitance provided to the chip logic (68) is equal to the implicit decoupling capacitance (74) between the signal line (70) and the ground line (54).

[0008] With reference to FIG. 2, it is apparent that even with a limited number of logic circuits (60) and (68), the number of connecting lines including power supply lines (48) and (52), ground lines (50) and (54), and signal lines (62) and (70) is numerous. As the number of logic circuits increase, generally, the number of connecting lines also increase. To manage the proper connection of power, ground, and/or signal lines, a program executed on a computer is used to route the connections, often called a “routing program” or “router”.

[0009] A database is used to maintain information necessary for the manufacture of an integrated circuit. The locations of the logic circuits, along with their connections to other circuits and power, are some of the information contained in the database. A routing program is responsible for making the appropriate connections between elements on the integrated circuit, often using a set of rules. The set of rules varies, but may include which signals are routed first, locations where a signal may or may not be routed, whether the signals must stay on a grid or can be freeform, width or the signal lines, spacing between signal lines, and which layers are allowed to be used in a multilayer connection arrangement.

SUMMARY OF INVENTION

[0010] According to one aspect of the present invention, a method for preferentially shielding a signal path having a value switchable between a first potential and a second potential comprises assigning a shield to shield the signal path in one of a plurality of routing channels, assigning the signal path in one of the plurality of routing channels, determining a probability of the value being at the first potential versus the second potential, and assigning a third potential to the shield depending on the probability.

[0011] According to another aspect, an integrated circuit comprises a signal having a value switchable between a first potential and a second potential, the value having a probability of being at the first potential versus the second potential; and a shield to shield the signal assigned to one of a plurality of possible routing channels, where the signal is assigned to one of the plurality of possible routing channels, and where the shield is assigned a third potential based on the probability.

[0012] According to another aspect, a computer system for preferentially routing a signal having a value switchable between a first potential and a second potential on an integrated circuit comprises a processor, a memory; and software instructions stored in the memory for enabling the computer system under control of the processor for determining a probability of the value being at the first potential versus the second potential; assigning a shield to shield the signal in one of a plurality of possible routing channels; and assigning the signal to one of the plurality of possible routing channels, where a third potential is assigned to the shield depending on the probability.

[0013] According to another aspect, a computer-readable medium having recorded thereon instructions executable by a processor, where the instructions are for using a probability of a signal value being at a first potential versus a second potential, assigning a shield to shield the signal to one of a plurality of possible routing channels, and assigning the signal to one of the plurality of possible routing channels, where the shield is assigned a third potential depending on the probability.

[0014] Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGs

[0015]FIG. 1 shows a typical computer system.

[0016]FIG. 2 shows a section of a typical computer system component.

[0017]FIG. 3 shows a circuit in accordance with an embodiment of the present invention.

[0018]FIG. 4 shows a circuit in accordance with another embodiment of the present invention.

[0019]FIG. 5 shows a flow process in accordance with some embodiments of the present invention.

[0020]FIG. 6 is a multilevel routing channel in accordance with an embodiment of the present invention.

DETAILEE DESCRIPTION

[0021] Embodiments of the present invention relate to a circuit that uses preferential shielding to increase implicit decoupling capacitance. Embodiments of the present invention further relate to a method for increasing implicit decoupling capacitance. Embodiments of the present invention further relate to a means for increasing implicit decoupling capacitance. Embodiments of the present invention further relate to a computer system that preferentially shields a wire to increase system performance. Embodiments of the present invention further relate to a means for increasing implicit decoupling capacitance by determining an appropriate preferential shield for a signal. Embodiments of the present invention further relate to a computer system that assigns a preferential potential to a shielding line to shield a signal to increase system performance. Embodiments of the present invention further relate to a computer-readable medium having recorded thereon instructions executed by a processor for increasing implicit decoupling capacitance by determining an appropriate preferred shielding potential to shield a signal.

[0022] In FIG. 2, during the course of normal operation of the computer system component (40), chip logic circuits (60) and (68) may generate a signal on signal lines (62) and (70) that statistically spends more time in either the high or low state. Depending on the statistically tendencies of the signal lines (62) and (70), the implicit decoupling capacitances (64) and (66) may not be equal, and the implicit decoupling capacitances (72) and (74) may not be equal, respectively.

[0023] As discussed above with reference to FIG. 2, a signal on the signal line may be high or low, and depending on the value of that signal, the amount of implicit decoupling capacitance present can be determined. In other words, the amount of implicit decoupling capacitance present on a signal line is a function of whether a signal on the signal line is high or low relative to a respective power supply or ground line. Regardless of whether a signal has a tendency to have a particular value, however, the amount of implicit decoupling capacitance present will be equal to the amount of implicit decoupling capacitance that would be present if the signal had another value. In other words, no preference is given in assigning the potential of the lines near the signal line. For example, although a NOR gate under normal conditions has a tendency to output low, the amount of implicit decoupling capacitance present will be the same for when the NOR gate outputs low and when the NOR gate outputs high.

[0024] The present invention takes into account a probability of a signal having a specific value. A signal line for the signal is routed amongst shields. By knowing the probability, a preferred potential is assigned to shield the signal line to maximize the implicit decoupling capacitance between the signal line and its shield. This technique assists in the assignment of the shield potential. By using the probability the signal is at a specific value, those skilled in the art will appreciate that the problem can be formulated to maximize the implicit decoupling capacitance by assignment of the shield potential.

[0025]FIG. 3 shows an exemplary circuit (140) in accordance with an embodiment of the present invention. The circuit (140) has a power supply bus (144) and a ground bus (146) that provides power to a two-input NOR gate (160) through a power supply line (148) and a ground line (150). The power supply (142) may be a part of the circuit (140) or a separate element. Power from the power supply (142) is made available to multiple power supply lines (148) and (152) via connections to the power supply bus (144) and to multiple ground lines (150) and (154) via connections to the ground bus (146). Power from the power supply (142) is delivered to chip logic circuits (160) and (168) via the power supply lines (148) and (152), respectively, and ground lines (150) and (154), respectively. The two-input NOR gate (160) outputs a signal on a signal line, or signal path, (162). The implicit decoupling capacitance is equal to the implicit decoupling capacitance (164) between the signal line (162) and the power supply line (148), acting as a first shield, plus the implicit decoupling capacitance (166) between the signal line (162) and the power supply line (152), acting as a second shield, while the signal line (162) is low. The implicit decoupling capacitance is essentially zero while the signal line (162) is high. This relationship may be represented as shown in Equation 1:

C _(IMP)=(Decap₁+Decap₂) when Signal Line=low, C _(IMP)=0 when Signal Line=high,  (Equation 1)

[0026] where Decap₁ is equal to the implicit decoupling capacitance (164) between the power supply line (148) and the signal line (162) when the signal line (162) is low, and where Decap₂ is equal to the implicit decoupling capacitance (166) between the power supply line (152) and the signal line (162) when the signal line (162) is low. When the signal line (162) is high, both the implicit decoupling capacitances (164) and (166) are essentially zero. From Equation 1, it is apparent that shielding the signal line (162) with power supply line (148) and power supply line (152) increases the amount of implicit decoupling capacitance. In fact, the longer signal line (162) is low, the greater the implicit decoupling capacitance, as shown in Equation 1. Equally stated, the average implicit decoupling capacitance over time is the probability the signal line (162) is low times the implicit decoupling capacitance when the signal line (162) is low, as defined in Equation 1. The average decoupling capacitance AvgC_(IMP) is:

AvgC _(IMP) =P _(L)*(Decap₁+Decap₂)  (Equation 2)

[0027] where P_(L) is equal to the probability that the signal is low. From Equation 2, it is apparent that in order to maximize the amount of implicit decoupling capacitance, the implicit decoupling capacitance is increased for the situation that has the higher probability on the signal line (162).

[0028] Of the four possible input combinations to the NOR gate (160), only one combination causes the NOR gate (160) to output high. The other three input combinations cause the NOR gate (160) to output low. Assuming an independent and evenly distributed probability for the inputs to the NOR gate (160), the probability that a signal outputted from the NOR gate (160) is high is 0.25 and the probability that a signal outputted from the NOR gate (160) is low is 0.75. Accordingly, the signal from the NOR gate (160) is said to be “predominantly low.”

[0029] In order to maximize the amount of implicit decoupling capacitance on the signal line (162) according to Equation 2, the signal line (162) is shielded such that both of the shielding lines (148) and (152) are connected to the power supply bus (144). The signal on the signal line (162) is three times more likely to be low; therefore, the implicit decoupling capacitance created by implicit decoupling capacitance (164) and implicit decoupling capacitance (166) exists 75% of the time and provides essentially no implicit decoupling capacitance only 25% of the time.

[0030] Those skilled in the art will appreciate that shield assignment, whether it is with a power supply line and a ground line, with a power supply line and a power supply line, or with a ground line and a ground line, the delay of the signal on the signal line does not change. Using preferential shielding, therefore, does not increase the signal delay compared to typical shielding arrangements. By creating an imbalance in the implicit decoupling capacitance based on the probability the signal is at a specific value, the amount of implicit decoupling capacitance is increased.

[0031] Those skilled in the art will appreciate that power supply line (148) and power supply line (152) act as shields. Further, those skilled in the art will appreciate that a different potential can be used on shielding line (148) and shielding line (152). Those skilled in the art will also appreciate that only one shielding line (148) or shielding line (152) may be used.

[0032] Still referring to FIG. 3, an exemplary circuit (140) is also provided with a two-input NAND gate (168) that receives power from the power supply bus (144) and from the ground bus (146) via power supply line (152) and ground line (154). The two-input NAND gate (168) outputs a signal on a signal line, or signal path, (170). The implicit decoupling capacitance is equal to the implicit decoupling capacitance (172) between the signal line (170) and the ground line (150), acting as a first shield, plus the implicit decoupling capacitance (174) between the signal line (170) and the ground line (154), acting as a second shield, while the signal line (170) is high. The implicit decoupling capacitance is essentially zero while the signal line (170) is low. This relationship may be represented as shown in Equation 3:

C _(IMP)=(Decap₁+Decap₂) when Signal Line=high, C _(IMP)=0 when Signal Line=low,  (Equation 3)

[0033] where Decap₁ is equal to the implicit decoupling capacitance (172) between the ground line (150) and the signal line (170) is high, and where Decap₂ is equal to the implicit decoupling capacitance (174) between the ground line (154) and the signal line (170) when the signal line (170) is high. When the signal line (170) is low, both the implicit decoupling capacitances (172) and (174) are essentially zero. From Equation 3, it is apparent that that shielding signal line (170) with ground line (150) and ground line (154) increases the amount of implicit decoupling capacitance. In fact, the longer signal line (170) is high, the greater the implicit decoupling capacitance, as shown in Equation 3. Equally stated, the average implicit decoupling capacitance over time is the probability the signal line (170) is high times the implicit decoupling capacitance when the signal line (170) is high, as defined in Equation 3. The average decoupling capacitance AvgC_(IMP) is:

AvgC _(IMP) =P _(H)*(Decap₁+Decap₂),  (Equation 4)

[0034] where P_(H) is equal to the probability that the signal is high. From Equation 4, it is apparent that in order to maximize the amount of implicit decoupling capacitance, the implicit decoupling capacitance is increased for the situation that has the higher probability on the signal line (170).

[0035] Of the four possible input combinations to the NAND gate (168), only one combination causes the NAND gate (168) to output low. The other three input combinations cause the NAND gate (168) to output high. Assuming an independent and evenly distributed probability for the inputs to the NAND gate (168), the probability that a signal outputted from the NAND gate (168) is low is 0.25 and the probability that a signal outputted from the NAND gate (168) is high is 0.75. Accordingly, the signal from the NAND gate (168) is said to be “predominantly high.”

[0036] In order to maximize the amount of implicit decoupling capacitance on the signal line (170) according to Equation 4, the signal line (170) is shielded such that both of the shielding lines (150) and (154) are connected to the ground bus (146). The signal on the signal line (170) is three times more likely to be high; therefore, the implicit decoupling capacitance created by implicit decoupling capacitance (172) and implicit decoupling capacitance (174) exists 75% of the time and provides essentially no implicit decoupling capacitance only 25% of the time.

[0037] Those skilled in the art will appreciate that shield assignment, whether it is with a power supply line and a ground line, with a power supply line and a power supply line, or with a ground line and a ground line, the delay of the signal on the signal line does not change. Using preferential shielding, therefore, does not increase the signal delay compared to typical shielding arrangements. By creating an imbalance in the implicit decoupling capacitance based on the probability the signal is at a specific value, the amount of implicit decoupling capacitance is increased.

[0038] Those skilled in the art will appreciate that ground line (150) and ground line (154) act as shields. Further, those skilled in the art will appreciate that a different potential can be used on shielding line (150) and shielding line (154). Those skilled in the art will also appreciate that only one shielding line (150) or shielding line (154) may be used.

[0039] By way of comparison, the average implicit decoupling capacitance in FIG. 2 and FIG. 3 are calculated assuming independent and evenly distributed probability for the inputs to all logic circuits, and the implicit decoupling capacitors are of equal value that are now referred to simply as “Decap”. For FIG. 2, the signal line (62) has a 0.25 probability of being high and a 0.75 probability of being low. Accordingly, the average implicit decoupling capacitance is 0.25 Decap to the ground line (50) and 0.75 Decap to the power supply line (48). Likewise in FIG. 2, the signal line (70) has a 0.25 probability of being low and a 0.75 probability of being high. Accordingly, the average implicit decoupling capacitance is 0.25 Decap to the power supply line (52) and 0.75 Decap to the ground line (54). Essentially, the power supply lines (48) and (52) combined see one Decap and the ground lines (50) and (54) combined see one Decap. For FIG. 3, the signal line (162) has a 0.25 probability of being high and a 0.75 probability of being low. Accordingly, the average implicit decoupling capacitance is 0.75*2*Decap, or 1.5 Decap, to power supply lines (148) and (152). Likewise in FIG. 3, the signal line (170) has a 0.25 probability of being low and a 0.75 probability of being high. Accordingly, the average implicit decoupling capacitance is 0.75*2*Decap, or 1.5 Decap, to ground lines (150) and (154). Those skilled in the art will appreciate that shielding the signal lines (162) and (170) with a preferential potential on the shielding lines in this manner increases the implicit decoupling capacitance associated with the signal lines (162) and (170).

[0040] Those skilled in the art will appreciate that although the embodiments shown in FIG. 3 use two-input logic gates, circuit logic having any number of inputs may be used, generally referred to as combinational logic. All that is required is that one be able to ascertain the probability of a value on an output signal from particular circuit logic. Further, those skilled in the art will appreciate that the embodiments shown in FIG. 3 may reside in an integrated circuit or other electrical component.

[0041]FIG. 4 shows an exemplary circuit (240) in accordance with an embodiment of the present invention. The circuit (240) has a power supply line (280) and a ground line (250) that provides power to, in this example, a two-input NOR gate (260). Power is provided to the power supply line (280) and the ground line (250) through a power supply (278). The power supply (278) may be a part of circuit (240) or a separate element. A power supply (242) also exists that supplies power through power supply line (248) and a ground line (250). The power supply (242) may be a part of circuit (240) or a separate element. In this embodiment, the ground line is common between both power supply (278) and power supply (242). Those skilled in the art will appreciate that the ground line (250) may actually be multiple lines that do not physically connect to one another. The two-input NOR gate (260) outputs a signal on a signal line, or signal path, (262). The implicit decoupling capacitance is equal to the implicit decoupling capacitance (264) between the signal line (262) and the power supply line (280), acting as a first shield, plus the implicit decoupling capacitance (266) between the signal line (262) and the power supply line (248), acting as a second shield, while the signal line (262) is low. The implicit decoupling capacitance is essentially zero while the signal line (262) is high. This relationship may be represented as shown in Equation 5:

C _(IMP)=(Decap₁+Decap₂) when Signal Line=low, C _(IMP)=0 when Signal Line=high,  (Equation 5)

[0042] where Decap₁ is equal to the implicit decoupling capacitance (264) between the power supply line (280) and the signal line (262) when the signal line (262) is low, and where Decap₂ is equal to the implicit decoupling capacitance (266) between the power supply line (248) and the signal line (262) when the signal line (262) is low. When the signal line (262) is high, both the implicit decoupling capacitances (264) and (266) are essentially zero. From Equation 5, it is apparent that shielding signal line (262) with power supply line (280) and power supply line (248) increases the amount of implicit decoupling capacitance. In fact, the longer signal line (262) is low, the greater the implicit decoupling capacitance, as shown in Equation 5. Equally stated, the average implicit decoupling capacitance over time is the probability the signal line (262) is low times the implicit decoupling capacitance when the signal line (262) is low, as defined in Equation 5. The average decoupling capacitance AvgC_(IMP) is:

AvgC _(IMP) =P _(L)*(Decap₁+Decap₂),  (Equation 6)

[0043] where P_(L) is equal to the probability that the signal is low. From Equation 6, it is apparent that in order to maximize the amount of implicit decoupling capacitance, the implicit decoupling capacitance is increased for the situation that has the higher probability on the signal line (262).

[0044] Of the four possible input combinations to the NOR gate (260), only one combination causes the NOR gate (260) to output high. The other three input combinations cause the NOR gate (260) to output low. Assuming an independent and evenly distributed probability for the inputs to the NOR gate (260), the probability that a signal outputted from the NOR gate (260) is high is 0.25 and the probability that a signal outputted from the NOR gate (260) is low is 0.75. Accordingly, the signal from the NOR gate (260) is said to be “predominantly low.”

[0045] In order to maximize the amount of implicit decoupling capacitance on the signal line (262) according to Equation 6, the signal line (262) is shielded such that both of the shielding lines (280) and (248) are connected to the power supplies (278) and (242), respectively. The signal on signal line (262) is three times more likely to be low; therefore, the implicit decoupling capacitance created by implicit decoupling capacitance (264) and implicit decoupling capacitance (266) exists 75% of the time and provides essentially no implicit decoupling capacitance only 25% of the time.

[0046] Those skilled in the art will appreciate that shield assignment, whether it is with a power supply line and a ground line, with a power supply line and a power supply line, or with a ground line and a ground line, the delay of the signal on the signal line does not change. Using preferential shielding, therefore, does not increase the signal delay compared to typical shielding arrangements. By creating an imbalance in the implicit decoupling capacitance based on the probability the signal is at a specific value, the amount of implicit decoupling capacitance is increased.

[0047] Still referring to FIG. 4, the power supply (278) connects to the implicit decoupling capacitance (264) through shielding line (280), and power supply (242) connects to the implicit decoupling capacitance (266) through shielding line (248). The power supplies (278) and (242) are not required to operate at the same potential. Consequently, the decoupling ability of the implicit decoupling capacitance (264) and the implicit decoupling capacitance (266) may not be same due to the stored charge differences. Those skilled in the art will appreciate that shielding the signal line (262) with one or more preferential potentials on the shielding lines in this manner increases the ability of the implicit decoupling capacitance associated with the signal line (262) to maintain the potential on the shielding lines. Those skilled in the art will also appreciate that only one shielding line (280) or shielding line (248) may be used.

[0048] Those skilled in the art will appreciate that although the embodiments shown in FIG. 4 use a two-input logic gate, circuit logic having any number of inputs may be used, generally referred to as combinational logic. Also, although a NAND gate is used for purposes of illustration, any type of logic gate may be used. All that is required is that one be able to ascertain the probability of a value on an output signal from particular circuit logic. Further, those skilled in the art will appreciate that the embodiments shown in FIG. 4 may reside in an integrated circuit or other electrical component.

[0049] In the design of an integrated circuit, connecting lines are used to connect one element to another. Due to the large number of connections that are needed, a routing program executed on a computer is sometimes used to appropriately create the required connections. FIG. 5 shows an exemplary flow process in accordance with an embodiment of the present invention. The flow process shows how an embodiment of the present invention assigns a preferred potential to the shielding lines to increase the implicit decoupling capacitance of a signal line. Power lines in this embodiment provide power between a power supply and combinational logic and are also used to shield lines. In step 302, the probability that signals on a signal line from the combinational logic are in a first state versus a second state is determined. In step 304, the signal lines are routed. In step 306, power lines are placed in the routing database such that they will provide shielding when their potential is assigned. Those skilled in the art will appreciate that steps (302, 304, 306) may be order independent or may occur in a sequential order.

[0050] Those skilled in the art will appreciate that the router may determine the routing of the signal lines then the power lines, the power lines then the signal lines, or the routing may occur in an interleaved method. Those skilled in the art will also appreciate that routing or manual placement or the signal lines and/or the power lines may occur. Those skilled in the art will further appreciate that the power lines may be placed and fixed before the signal lines are routed. Those skilled in the art will additionally appreciate that the signal lines may be routed before the power lines are routed, in which case, the routing program may automatically reposition the signal lines as the power lines are added.

[0051] In step 308, the power lines are assigned a potential based on the probability that the signals on the signal line are in the first state versus the second state.

[0052] Those skilled in the art will appreciate that the placement of the signal line and/or the power lines may or may not be required to adhere to a grid. Also, the placement of the signal line may include additional rules that constrain the location of the routing. Further, those skilled in the art will appreciate that the steps shown in FIG. 5 refer to the method of creating the information, or database, necessary to manufacture an integrated circuit or other electrical component.

[0053] Another tool used with a database that includes information for the manufacture of an integrated circuit or other electrical component is a computer system and a set of instructions, or program, to display the information in a graphical format. FIG. 6 shows an exemplary display of a routing section (400) in accordance with an embodiment of the present invention. The routing section (400) has a first power supply bus (444) and a second power supply bus (446) that can provide power to a combinational circuit or circuits (not shown) via routed power supply lines (402, 404, 406, 408). The first power supply bus (444) and the second power supply bus (446) are routed in a layer below the layer used to route the power supply lines (402, 404, 406, 408). The “X” indicates that a connection is made between the lower layer and upper layer, e.g., connection (480). The routing section (400) does not currently have any assigned connections between the first power supply bus (444) and the power supply lines (402, 404, 406, 408), or the second power supply bus (446) and the power supply lines (402, 404, 406, 408).

[0054] Still referring to FIG. 6, placeholders for a combinational logic circuit (420) and a combinational logic circuit (430) display the location of the combinational logic, but do not show any details about the logic elements within the combinational logic (420) and the combinational logic circuit (430). In this example, the combinational logic (420) has three combinational logic outputs (422, 424, 426), and the combinational logic (430) has two combinational logic outputs (432, 434). The combinational logic outputs (422, 424, 426, 432, 434) are routed in the same layer as the layer used to route the power supply bus (444) and the power supply (446).

[0055] The combinational logic outputs (422, 424, 426, 432, 434) are connected to the signal routing lines (452, 454, 456, 462, 464), respectively. A combinational logic output (426) is also connected to signal routing line (458). The combinational logic outputs (422, 424, 426, 432, 434) are routed in the layer below the layer used to route the signal routing lines (452, 454, 456, 458, 462, and 464). The “X” indicates that a connection is made between the lower layer and upper layer, e.g. connection (480).

[0056] The power supply lines (402, 404, 406, 408) are positioned to provide shielding to the signal routing lines (452, 454, 456, 458, 462, 464). The routing program must make a connection from the first power supply bus (444) or the second power supply bus (446) to the routed power supply lines (402, 404, 406, 408), depending on the probability that the output from each of the combinational logic outputs (422, 424, 426, 432, 434) are in a first state versus a second state. For example, the preferential potential applied to power supply lines (404, 406) depends on the probability that the combinational logic output (426), hence signal routing lines (456, 458), is in the first state versus the second state.

[0057] The preferential potential applied to power supply line (402) and power supply line (404) is determined by the probability that the combinational logic output (426) and the combinational logic output (432) are in the first state versus the second state. The preferential potential applied to the power supply line (402) and power supply line (404) depends on which potential provides more implicit decoupling capacitance. Due to the router generated arrangement of signal routing lines (456) and (462), both have an effect on the determination of the preferred potential. Those skilled in art will appreciate that implicit decoupling capacitance is determined in part by the routing length and spacing of a signal line and/or power supply line, and that the implicit decoupling capacitance value may also be a deciding factor along with the probability of the signal on the signal line being in a first state versus a second state when two or more combinational logic outputs prefer a different shielding potential and share a common shielding line.

[0058] Still referring to FIG. 6, power supply lines (402, 404, 406) are positioned between two signal lines that may each have a different preferred potential. For example, power supply line (402) shields both the signal routing line (452) and the signal routing line (462). The preferential potential applied to the power supply lines (402, 404, 406) depends on which potential provides more implicit decoupling capacitance. Those skilled in art will appreciate that implicit decoupling capacitance is determined in part by the routing length and spacing of a signal line and/or power supply line, and that the implicit decoupling capacitance value may also be a deciding factor along with the probability of the signal on the signal line being in a first state versus a second state when two or more combinational logic outputs prefer a different shielding potential and share a common shielding line.

[0059] Those skilled in art will appreciate that the first power supply bus (444) and the second power supply bus (446) may have different potentials, which include ground as a potential. Those skilled in art will also appreciate that signals and/or power can be transmitted on multiple routing layers and still achieve the desired preferential shielding. Further, those skilled in art will appreciate that power supply lines may be assigned based on an ordered, or prioritized list, in combination with the probability of the signal on the signal line being in the first versus second state.

[0060] Those skilled in art will appreciate that a method is described to route the power supply lines, that both transmits power and acts as a shielding element, along with the signal lines, then assign the preferred potential to the power lines based on the probability of the signal line being in a first state versus a second state to increase implicit decoupling capacitance on an integrated circuit or electrical component.

[0061] Those skilled in art will appreciate that a computer system is described to route the power supply lines, that both transmits power and acts as a shielding element, along with the signal lines, then assign the preferred potential to the power lines based on the probability of the signal line being in a first state versus a second state to increase implicit decoupling capacitance on an integrated circuit or electrical component.

[0062] The invention also may be embodied in a computer-readable medium having recorded thereon instructions executable by a processor is described to route the power supply lines, that both transmits power and acts as a shielding element, along with the signal lines, then assign the preferred potential to the power lines based on the probability of the signal line being in a first state versus a second state to increase implicit decoupling capacitance on an integrated circuit or electrical component.

[0063] Those skilled in art will appreciate that a power supply may transmit any potential, including ground.

[0064] Advantages of the present invention may include one or more of the following. In some embodiments, because a signal is preferentially shielded, a circuit may have more decoupling capacitance than when the signal is not shielded by a preferred potential.

[0065] In some embodiments, because a signal is shielded with a preferred potential, increased decoupling capacitance is provided to one or more components in a computer system, effectively leading to better system performance.

[0066] In some embodiments, because implicit decoupling capacitance is increased through preferential shielding, fewer explicit decoupling capacitors are needed, and thus, more components may be positioned on an integrated circuit.

[0067] In some embodiments, because implicit decoupling capacitance is higher, the number of explicit decoupling capacitors that must be added is reduced; therefore, a smaller integrated circuit die may be used.

[0068] While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

What is claimed is:
 1. A method for preferentially shielding a signal path having a value switchable between a first potential and a second potential, comprising: assigning a shield to shield the signal path in one of a plurality of routing channels; assigning the signal path in one of the plurality of routing channels; determining a probability of the value being at the first potential versus the second potential, and assigning a third potential to the shield depending on the probability.
 2. The method of claim 1, wherein the third potential is closer in value to the first potential than to the second potential when the probability of the value of the signal being at the second potential is greater.
 3. The method of claim 1, wherein the third potential is closer in value to the second potential than to the first potential when the probability of the value of the signal being at the first potential is greater.
 4. The method of claim 1, wherein the shield is operatively connected to a power supply.
 5. The method of claim 1, wherein the signal path is operatively connected to a logic component.
 6. The method of claim 1, wherein the first potential is a logic low and the second potential is a logic high.
 7. The method of claim 6, wherein the third potential is ground.
 8. The method of claim 6, wherein the third potential is power.
 9. An integrated circuit, comprising: a signal having a value switchable between a first potential and a second potential, the value having a probability of being at the first potential versus the second potential; and a shield to shield the signal assigned to one of a plurality of possible routing channels; wherein the signal is assigned to one of the plurality of possible routing channels; and wherein the shield is assigned a third potential based on the probability.
 10. The integrated circuit of claim 9, wherein the third potential is closer in value to the first potential than to the second potential when the probability of the value of the signal being at the second potential is greater.
 11. The integrated circuit of claim 9, wherein the third potential is closer in value to the second potential than to the first potential when the probability of the value of the signal being at the first potential is greater.
 12. The integrated circuit of claim 9, wherein the shield is operatively connected to a power supply.
 13. The integrated circuit of claim 9, wherein the signal path is operatively connected to a logic component.
 14. The integrated circuit of claim 9, wherein the first potential is a logic low and the second potential is a logic high.
 15. The integrated circuit of claim 14, wherein the third potential is ground.
 16. The integrated circuit of claim 14, wherein the third potential is power.
 17. A computer system for preferentially routing a signal having a value switchable between a first potential and a second potential on an integrated circuit, comprising: a processor; a memory; and software instructions stored in the memory for enabling the computer system under control of the processor, for: determining a probability of the value being at the first potential versus the second potential; assigning a shield to shield the signal in one of a plurality of possible routing channels; and assigning the signal to one of the plurality of possible routing channels; wherein a third potential is assigned to the shield depending on the probability.
 18. The computer system of claim 17, wherein the third potential is closer in value to the first potential than to the second potential when the probability of the value of the signal being at the second potential is greater.
 19. The computer system of claim 17, wherein the third potential is closer in value to the second potential than to the first potential when the probability of the value of the signal being at the first potential is greater.
 20. The computer system of claim 17, wherein the shield is operatively connected to a power supply.
 21. The computer system of claim 17, wherein the signal path is operatively connected to a logic component.
 22. The computer system of claim 17, wherein the first potential is a logic low and the second potential is a logic high.
 23. The computer system of claim 22, wherein the third potential is ground.
 24. The computer system of claim 22, wherein the third potential is power.
 25. A computer-readable medium having recorded thereon instructions executable by a processor, the instructions for: using a probability of a signal value being at a first potential versus a second potential; assigning a shield to shield the signal to one of a plurality of possible routing channels; and assigning the signal to one of the plurality of possible routing channels; wherein the shield is assigned a third potential depending on the probability.
 26. The computer-readable medium of claim 25, wherein the third potential is closer in value to the first potential than to the second potential when the probability of the value of the signal being at the second potential is greater.
 27. The computer-readable medium of claim 25, wherein the third potential is closer in value to the second potential than to the first potential when the probability of the value of the signal being at the first potential is greater.
 28. The computer-readable medium of claim 25, wherein the shield is operatively connected to a power supply.
 29. The computer-readable medium of claim 25, wherein the signal path is operatively connected to a logic component.
 30. The computer-readable medium of claim 25, wherein the first potential is a logic low and the second potential is a logic high.
 31. The computer-readable medium of claim 30, wherein the third potential is ground.
 32. The computer-readable medium of claim 30, wherein the third potential is power. 